1. Field of the Invention
The present invention relates to a bonded member comprising different types of materials and a production method thereof. More specifically, the present invention relates to a bonded member comprising different types of materials which is usable at high temperatures and a production method thereof.
2. Description of the Related Art
To bond different types of materials such as a ceramic base material and a metal member to each other, a method using a soldering material can be used. However, during a cooling process in high-temperature bonding, since thermal stress is generated due to the difference in thermal expansion coefficient between the different materials or between the soldering material used for bonding the different materials together and the materials, cracks are formed in the vicinity of the interface when one of the members is brittle, or the separation occurs at the interface between the members. Accordingly, the desired bonding strength and air tightness may not be able to be attained in some cases. Since products having aforementioned troubles broken during a production process must be discarded as defective products, this results in an unfavorable increase in production cost of these composite members. Further, if the product is subjected to thermal cycle in use, these troubles often occur after use for a certain time period to cause deterioration of reliability of the product.
As a method for bonding different materials together by use of a soldering material, a metallization bonding method and a direct bonding method are available. The metallization bonding method is a method in which a metal material is bonded to a surface of ceramic by forming a metal layer on the ceramic surface by a vapor phase process such as deposition from a vapor phase, vapor deposition or sputtering or a process in which a paste is applied to the ceramic surface which is then heated. As the direct bonding method, an active metal method is well-known. This is a method in which a metal material is bonded to a surface of ceramic by use of an element in the group IV as an insert material. Meanwhile, in these methods, unless some measures are taken against thermal stress which occurs at the interface between the bonded materials, the ceramic base material which is vulnerable to the thermal stress may often be cracked or separation may occur at the interface, so that bonding strength as well as various properties required as a composite member such as air tightness may be influenced. Particularly, it is very difficult to bond a low-strength member such as aluminum nitride to a different material such as a metal material with the above problems inhibited from occurring.
To solve the above problems, a method of bonding the base material and the metal member together through liquid-phase bonding using a soldering material comprising a low-strength metal such as Ag, Cu or Au which undergoes plastic deformation by low stress can be conceived. However, in a case where Ag or Cu is used, they may be difficult to use in some cases due to a problem caused by oxidation of the soldering material when a bonded member is exposed to an application environment of 400° C. or higher under oxidizing atmosphere, a problem caused by volatilization of the soldering material caused by their high vapor pressures when they are used at a high temperature and a low pressure, a problem of migration when energized and used, or a problem of a reaction with an Mo oxide. Meanwhile, use of Au is effective for avoiding the problems occurring when Ag or Cu is used. However, when Ni, Co, Kovar or other metal is used as a metal material having excellent oxidation resistance, its constituents (Fe, Ni, Co) are diffused into Au when the metal material is heated, thereby increasing the strength of the soldering material. As a result, residual stress at the time of bonding cannot be absorbed sufficiently by plastic deformation of the soldering material, and bonding strength lowers due to heat applied upon or after completion of the bonding. Consequently, when the bonded member undergoes a thermal cycle or thermal shock is given thereto, the ceramic base material may be cracked.
Further, it is known that when an Au-18Ni soldering material and a conductive material (Mo) are bonded together, Ni in the soldering material reacts with Mo and forms a brittle structure. Accordingly, when use of a resulting binding component as a member for a high-temperature heater is considered, there arises a problem that durability of the component when the interface between bonded members is exposed to a thermal cycle and thermal shock lowers and sharply deteriorates, thereby preventing further use of the component.
Further, when kovar is used as the metal material, for example, components (Fe, Ni, Co) constituting Kovar are diffused into the soldering material when the base material and Kovar are bonded together and form an intermetallic compound having low conductivity, so that there arise problems of deterioration of thermal properties and occurrence of abnormal heat generation at the site.
Meanwhile, use of metal not being solid soluble in Au as a metal material may also be possible. Illustrative examples of metal materials capable of meeting such requirement mentioned above may include W and Mo. However, these metal materials have a problem that they are oxidized vigorously at a high temperature in air and cannot be used as a metal material for a high-temperature heater exposed to conditions such as mentioned above.
To solve the above problem, an attempt to use a special design as a bonded structure has been made. For example, in JP-A-209255/1998, a bonded structure of a ceramic base material and a connector for power supply according to the structure shown in FIG. 4 is disclosed as a susceptor for mounting a semiconductor wafer. In FIG. 4, a hole 14 is formed in a ceramic base material 1. From the hole 14, a metal member 17 such as Mo which is embedded in the ceramic base material 1 in advance and has a thermal expansion coefficient close to that of the ceramic base material 1 is exposed. Further, in the hole 14, a cylindrical atmosphere protecting member 10 is inserted. In the atmosphere protecting member 10, a connector 16 for supplying power and a low heat expanding member 15 for alleviating stress are inserted. The atmosphere protecting member 10 and the connector 16 are hermetically bonded to each other by means of a soldering material 5, and the low heat expanding member 15 and the atmosphere protecting member 10 are hermetically bonded to the metal member 17 by means of the soldering material 5.
According to this bonded structure, residual stresses upon bonding the low heat expanding member 15 and the metal member 17 are alleviated, and oxidation of the metal member 17 such as Mo is inhibited by the atmosphere protecting member 10. Thus, even if they are bonded by means of a high strength soldering material such as the foregoing Au-18Ni, no cracks occurs in the ceramic base material at the time of bonding, a change in strength due to heat is small, and endurance reliability when bonded portions are exposed to a thermal cycle and thermal shock upon use of a high temperature heater is also high. However, the bonded structure has such problems that the number of components is large and that if the atmosphere protecting member 10 and the metal member 17 are not fully bonded to each other, deterioration occurs due to oxidation of the metal member 17, so that a very high production control ability is required.
Further, JP-A-11-278951 discloses, as a susceptor for mounting a semiconductor wafer, a bonded member adopting such a member structure as one shown in FIG. 6 or 7 so as to alleviate thermal stress which occurs when a corrosion-resistant metal ring 23 such as Kovar is bonded to a back side 22b of a ceramic susceptor 22 in a ceramic base material associated with the structure shown in FIG. 5, and a production method thereof. However, although adoption of such a member structure is effective for alleviation of thermal stress, since a Cu-based soldering material is used, usable temperatures are limited due to such problems as oxidation of the soldering material and evaporation of the soldering material caused by a vapor pressure.
To avoid the above problems, the present inventors have proposed a solid-phase bonded component formed by bonding a metal member comprising Ni to another member by means of a soldering material comprising Au, as disclosed in Japanese Patent Application No. 2000-227291. According to a method disclosed in the application, a brittle member and a member of different type can be bonded together with high reliability. However, when the solid-phase bonded material comprising the Au soldering material and the Ni metal member is kept in a high-temperature atmosphere exceeding 750°C. for a long time, a phenomenon that Ni is gradually diffused into Au and the strength of Au thereby increases in observed. Consequently, when a composite member which comprises a ceramic base material having an Au soldering material as a precoat layer and a metal member comprising Ni or the like is used as a member for an electrical apparatus exposed to temperatures exceeding 700°C. upon use, for example, a susceptor for mounting a semiconductor wafer, a problem that the ceramic base material cracks within a relatively short period is liable to occur.
Further, it is known that Pt is diffused in Au slowly, and JP-A-2001-199775 discloses a method in which ceramic and metal are bonded together by means of a soldering material composed essentially of Au and the metal is coated with Pt as a barrier layer so as to prevent diffusion of the metal component. However, when Ti is used for bonding ceramic to a soldering material, Pt cannot be used as a barrier layer. For example, when ceramic and a metal material coated with Pt are bonded together by use of Au and Ti, bonding between Au and ceramic does not occur effectively when liquid phase bonding disclosed in JP-A-199775/2001 is carried out. It is understood that this is because Ti fused into Au is drawn to the Pt layer due to good affinity between Pt and Ti, and resultantly this causes the shortage of Ti to be used for the formation of a TiN reaction layer at the interface. Accordingly, the formation of the TiN layer is unsatisfactory and bonding strength between the Au soldering material and ceramic is lowered.
Further, even when a Pt coating material is solid-phase bonded to an Au precoat layer using Ti as an active material in accordance with the method disclosed in the prior application laid-open as Japanese Patent Application No. 2000-47291 by the present inventors, a trace amount of Ti in an Au soldering material is drawn to Pt during the period of using such a bonded member at a temperature equal to or higher than a specific temperature, thereby bonding strength between ceramic and an Au soldering material layer is lowered. Furthermore, the bonding strength is also lowered due to Karkendal voids occurring in the vicinity of the interface of the Pt coating material upon movement of these atoms. Therefore, this method is difficult to be employed for the production of bonded members for certain use.